1. Technical Field
This invention generally relates to methods for inhibiting the oxidation of metal sulfide-containing material to reduce the amount of acid mine drainage (i.e., acidic wastewater) produced in mining and mineral processing operations and to metal sulfide-containing material having thereon an oxidation inhibiting lipid coating comprised of a lipid composition containing at least one or more lipid compounds having hydrophilic head groups and two of the same or different organic hydrophobic groups attached to each of the hydrophilic head groups dispensed in an aqueous medium.
2. Description of Related Art
There is considerable interest in the development of methods to reduce the amount of metal sulfide oxidation during mining and milling operations. Presently, the production of acidic drainage from reactive sulfide tailings and waste rock deposition produced during mining and milling of sulfide-containing ores is a major environmental and ecological problem. This is known as acid mine drainage (AMD) which is the phenomenon of acid production from mining tailings and frequently contains toxic metals. It results from the mining of coal and other metal sulfide-containing materials, e.g., metal sulfide-containing rock. Metal sulfide-containing rocks are commonly mined for their content of precious metals (e.g., platinum, gold, and silver) and base metals (zinc, lead, copper). Both working and defunct mines have AMD problems associated with them. AMD is a major environmental problem since a significant amount of the waterways such as rivers, streams and lakes are either adjacent to, or in close proximity to, the affected mining site.
In general, AMD is produced when the metal sulfide minerals, e.g., pyrite (FeS2), and ferric iron are exposed to the atmosphere. Upon contact with oxygen and water, the metal sulfide minerals undergo oxidation. This oxidation produces highly acidic water enriched with various heavy metals. For example, pyrite has a significant concentration in coal and ore mine waste rock such that upon oxidation the oxidized pyrite produces ferrous sulfate and sulfuric acid by a complex series of chemical reactions. This, in turn, results in the acidification of surface water (through the formation of sulfuric acid) and subsequent mobilization of toxic metals initially incorporated into the pyrite structure.
The high acidity and presence of toxic metals in AMD-containing waters degrade soil, air and water quality while also detrimentally impacting vegetation and aquatic life. Consequently, mine wastewaters, prior to being released into the environment, must be treated to meet government standards for the amount of metal and non-metal ions contained in the water. Some of these metals such as, for example, uranium and selenium, cause deleterious health effects and are extremely difficult to remove from mine wastewaters.
Past efforts to treat mine wastewaters have been ineffective or prohibitively expensive. When treatments are ineffective at removing some metal and non-metal ions, mining throughput can be restricted by governmental regulation. On the other hand, because other treatments are expensive, no cleanup has occurred in many cases, particularly for abandoned mines. Accordingly, up until the development of the present invention, there has been no reliable, long term, economic solution to reduce or prevent oxidation of metal sulfides such as pyrite that result in AMD.
One approach to preventing or controlling oxidation and therefore limit AMD was to encapsulate the pyrite mineral with a surface precipitate, such as iron phosphate or silica precipitates, designed to form a physical barrier for oxidants approaching its surface (See, e.g., Evangelou, V. P., “Potential microencapsulation of pyrite by artificial inducement of ferric phosphate coatings”, J. Environ. Qual., Vol. 24, pp. 535–542 (1995); and Zhang et al., “Formation of ferric hydroxide-silica coatings on pyrite and its oxidation behavior”, Soil Science, Vol. 163 (1), pp. 53–62 (1998)). Another approach was to complex aqueous Fe3+, since this species together with aqueous O2 has been shown to strongly oxidize pyrite (See, e.g., Singer et al., “Acidic mine drainage: The rate-determining step”, Science, Vol. 167, pp. 1121–1123 (1970)). Humic acid and other organic ligands, some leached from wood chips and manure, have been explored in this context (See, e.g., Lalvani et al., “Coal pyrite passivation due to humic acids and lignin treatment”, Fuel Science and Technology Int'l., Vol. 14(9), pp. 1291–1313 (1996); and Peiffer et al, “The oxidation of pyrite at pH 7 in the presence of reducing and nonreducing Fe (III)-chelators”, Geochimica et Cosmochimica Acta, Vol. 63, pp. 3171–3182 (1999); and Backes et al., “Studies on the oxidation of pyrite in colliery spoil II. Inhibition of the oxidation by amendment treatments”, Reclamation and Revegetation Research, Vol. 6, pp. 1–11 (1987)).
At least one problem with these approaches has been that at low pH the encapsulation or complexation approaches generally do not work well, because the protecting phase becomes soluble and is not stable for long times. In some cases, especially in the phosphate case, the approach itself is harmful to the environment and would pose additional problems in their use if the pH were to drop. For example, phosphate is known to be a primary cause of eutrophication in streams and particularly in ponds and lakes. Additionally, the phosphate coating on the iron sulfide has been found to be stable only at higher pH values, i.e., pH values greater than 4.0 (See, e.g., Elsetinow et al., “Aqueous geochernical and surface science investigation of the effect of phosphate on pyrite oxidation”, Environmental Science and Technology, Vol. 35, pp. 2252–2257 (2001)). However, at low pH values such as those in the range of a pH of about 2.5–4.0 that are prevalent in mining locations and spoil areas lead to a breakdown of the phosphate coating within about fifty days. Accordingly, unless the mining location and spoil area are periodically treated (e.g. about every thirty days) with limestone or other alkaline material to control pH and maintain the pH level between a pH of about 6 to about 8, the coating degrades and exposes the iron sulfide to oxidation. This, of course, leads to the gradual development of the acid solutions enriched with heavy metals that it hoped to avoid. Limestone buffering has also been shown to lead to increased pyrite oxidation (see, e.g., Evangelou et al., “Potential role of bicarbonate during pyrite oxidation”, Environmental Science and Technology, Vol. 32, pp. 2084–2091 (1998)).
It would therefore be desirable to provide a method for inhibiting the oxidation of metal sulfide-containing material to prevent or reduce the formation of AMD, which possesses a long term effect and is also stable at low pH values.